Many mechanical systems, such as vehicles, require liquids for their operation. To extend the life of the equipment, such liquids must be maintained, including the replacement of spent and degraded liquids.
Most systems are maintained on a scheduled basis, which is not dependent upon the actual condition of the liquid, but rather, is based upon the general understanding of the liquid's life. For example, most drivers change the oil in their vehicles at predefined mileage intervals, such as every 3000 miles. Although this is beneficial, it fails to account for other factors that may affect the condition of the liquid. For example, a failure in the equipment could cause the liquid to degrade at a faster rate than that anticipated by the normal replacement schedule. Similarly, the condition of the equipment might be such that the liquid's useful life is extended. Thus, costs can be increased due to poorly operating equipment or due to unnecessary liquid replacement. If the equipment requiring maintenance is a fleet, the expense could be quite large.
Currently, electrochemical sensor systems exist that allow the monitoring and measurement of the condition, or health, of such liquids. Indeed, many monitoring systems exist which measure a variety of liquid parameters, including, dielectric constant, conductivity, pH and the amount of water in the liquid. Although such measurements are useful, taken in isolation they do not necessarily identify the health of the liquid unless the user is also aware of the measurement's history. For example, if the value of a particular parameter rises or falls as the liquid degrades, the liquid's health would be unclear from a single measurement. Rather, the parameter's history would be required to accurately assess the liquid condition.
One solution to the problems presented by electrochemical measurements is to measure liquid viscosity that, by itself, can be a good indicator of liquid health. The measurement of viscosity does not suffer from the historical problems associated with electrochemically measured parameters of the liquid. Indeed, if viscosity increases monotonically with operating time and, if a liquid's viscosity degradation profile is known in advance, then its health can be determined by a single measurement regardless of when the measurement is taken.
As viscosity is not directly measured by electrochemical sensors, but rather, is measured by the application of mechanical forces, including compressive forces and shear forces, a separate measuring sensor must be used. One approach uses a vibrating quartz or piezoelectric element that measures the shift in a device's resonant frequencies in response to applied vibrations, which in turn is a measurement of the damping value Q and thus of viscosity. The measurement of Q, however, is not a linear measurement, and thus will not be useful for a wide viscosity range. Further, this manner of measuring viscosity introduces complexities because it applies both compressive and shear forces. The contributions of both components to the net response can complicate data interpretation and limit operating range.
A more desirable and accurate viscosity measurement is obtained with a shear force measurement. One shear force measurement technique involves the dropping of large balls through cylinders filled with the liquid to be measured. As the ball moves through the liquid, the shear force resulting between the moving balls and the liquid can be measured. Although this technique is accurate, it is riot useful for smaller systems or equipment, and thus, is limited in its applications.
As seen from above, although viscosity measurements are desirable, current liquid monitoring systems require separate electrochemical and viscosity sensors to monitor both liquid health, and also the other liquid characteristics available from an electrochemical sensor. Indeed, most users will not simply replace their electrochemical sensors and rely solely on a viscosity measurement.
A need in the industry exists for a measurement system that provides the ability to measure the health of a liquid in a system, wherein the measuring system resides within the system. A further need in the industry exists for a viscosity sensor that utilizes a shear force measurement technique that can be used in small or confined environments, and can be combined with other sensors.